Brothers, if anyone is caught in any transgression, you who are spiritual should restore him in a spirit of gentleness. Keep watch on yourself, lest you too be tempted. Bear one another's burdens, and so fulfill the law of Christ. For if anyone thinks he is something, when he is nothing, he deceives himself. But let each one test his own work, and then his reason to boast will be in himself alone and not in his neighbor. For each will have to bear his own load.
What if you see someone sinning, what do you do? This is such a loaded question, because it does not define sin (transgression). For some, it would mean taking a drink of alcohol, or going to an "R" rated movie, or wearing a shirt that shows to much cleavage or smoking a cigarette, or questioning something that the preacher has said. I am not so sure that is the kind of transgressions this is speaking of. It is not about being the "Spiritual police." In fact, it goes on to say that "you who are spiritual should restore him." Wow! Talk about a passage that is open for abuse! Didn't the Pharisees consider themselves spiritual? What does it mean, "you who are spiritual? And notice that even the spiritual are open to temptation.
What I think this means is that if we see someone who is doing something that goes against the fruit of the Spirit, someone who is abusive or thinks that their freedom in Christ gives them the right to behave in any manner, then one who understands the truth of the gospel should take them under their wing and seek to bring them back to the truth of the gospel in a loving and caring manner. The caution here is a valid one, because in doing this, you might be tempted and pulled away by their sin, by their lifestyle. This is Christianity, that we are not just concerned about our own salvation, but that we care for our brothers as well. We bear their burdens as our own, we place ourselves in their shoes, because that is what Christ did for us. We fulfill the law of Christ in doing this because it shows that we love one another.
But don't get the big head. Wearing the title "spiritual" (or deacon, elder, pastor, etc.) does not make you anything special in the eyes of God. We are all of Christ, all a part of the body, and no one part can function or exist on its own, so don't think of yourself as so important. If you do, you are only fooling yourself.
So test your work, evaluate your purpose, and remember that when you will answer to God it will be for your choices, and not what your brother has done or how they have responded to you.